"I lost almost 30 pounds," he told the magazine. "It really changes your face when you're gaunt like that. That really played well for the character, sort of high cheekbones and your eyes become even bigger. So I'm happy that I did it. But it wasn't always fun."

To lose so much weight, Skarsgård cut carbs from his diet a few months before filming. He then went more strict in the three weeks leading up to start.

"There's this thing called ketogenic diet. Essentially, you don't eat any carbs whatsoever," he said. "You eat protein and high fat. And once your body starts using fat as fuel, as opposed to carbs, you burn fat like crazy. And then I would do a couple of days of fasting, stuff like that."

Bill Skarsgård as the prisoner. Patrick Harbron/Hulu

"Castle Rock" is based on Stephen King's literary works. Skarsgård plays a mysterious Shawshank prisoner discovered in the depths of the prison with no identity or criminal record.